9

Usually when creating certain sequences of numbers, python and numpy offer some syntactic sugar to do so in a simple way, without generating them yourself with for-loops, e.g. range(start, stop, step).

I'm having a rather simple problem that I'm struggling to solve in an elegant way: Generate a list of the powers of two. E.g. list = [1, 2, 4, 8, ...].

I came up with

n_powers = 4
list = np.zeros(n_powers)
for i in range(0, n_powers): 
    list[i] = 2 ** i

Is there a better way to do this?

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3
  • 1
    "Better" how? What shortcomings do you observe in the solution you have now? Commented Mar 9, 2018 at 18:49
  • 2
    list is not a list it is a numpy.ndarray. These are two different data-structures that typically have different idiomatic approaches to doing things. If your list variable were indeed a list object, this would be a perfectly reasonable way of doing it, except you would normally use .append on an empty list, and not pre-allocate it. Commented Mar 9, 2018 at 18:50
  • Also don't use list as a variable name because your overwrite the built-in type list then. Commented Mar 9, 2018 at 18:53

6 Answers 6

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19

You seem to be using NumPy, so why not just do this -

>>> 2 ** np.arange(4)
array([1, 2, 4, 8])

This is broadcasted exponentiation.

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4 Comments

Cool, something like this is what I was looking for - my mistake was not using numpy before. I tried: list = range(0,4) and then list = 2 ** list gave me an error TypeError: unsupported operand type(s) for ** or pow(): 'list' and 'int'
@Honeybear Yes, because pure python lists do not support vectorized arithmetic.
2 ** np.array() vs np.array() ** 2 gives different results
well, yes, if you think about it, it makes perfect sense. The first expression raises 2 to the power of the elements in the array, the second expression raises the elements in the array to the power of 2. @Edi
8

Maybe just:

l = [2**i for i in range(n)]
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2 
limit = int(input('Limit: '))
l = []

for i in range(limit):
    l.append(2**i)

It is also good to not use list as a variable as is is a datatype and has a built-in list() function

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2

As per the documentation, you can use power() function to increase to desired power in numpy.

i.e. to increase in the power of 2 you can use this: np.power(np.arange(4), 2)

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1

I am not sure what your question is You can just do:

n_powers = 4
list = []
for i in range(n_powers):
    list.append(2 ** i)
print(list)
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1 Comment

as @Crawley said, it is better not to use list as a variable but it still works.
0

In the case of this problem (powers of 2) you can use the following Numpy method.

np.exp2(np.arange(4))

Out: array([1., 2., 4., 8.])
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